Synthesis of two forms of apolipoprotein B by cultured rat hepatocytes

Model systems for studying the assembly, trafficking, and secretion of apoB lipoproteins using fluorescent fusion proteins

apoB exists as apoB100 and apoB48, which are mainly found in hepatic VLDLs and intestinal chylomicrons, respectively. Elevated plasma levels of apoB-containing lipoproteins (Blps) contribute to coronary artery disease, diabetes, and other cardiometabolic conditions. Studying the mechanisms that drive the assembly, intracellular trafficking, secretion, and function of Blps remains challenging. Our understanding of the intracellular and intraorganism trafficking of Blps can be greatly enhanced, however, with the availability of fusion proteins that can help visualize Blp transport within cells and between tissues. We designed three plasmids expressing human apoB fluorescent fusion proteins: apoB48-GFP, apoB100-GFP, and apoB48-mCherry. In Cos-7 cells, transiently expressed fluorescent apoB proteins colocalized with calnexin and were only secreted if cells were cotransfected with microsomal triglyceride transfer protein. The secreted apoB-fusion proteins retained the fluorescent protein and were secreted as lipoproteins with flotation densities similar to plasma HDL and LDL. In a rat hepatoma McA-RH7777 cell line, the human apoB100 fusion protein was secreted as VLDL- and LDL-sized particles, and the apoB48 fusion proteins were secreted as LDL- and HDL-sized particles. To monitor lipoprotein trafficking in vivo, the apoB48-mCherry construct was transiently expressed in zebrafish larvae and was detected throughout the liver. These experiments show that the addition of fluorescent proteins to the C terminus of apoB does not disrupt their assembly, localization, secretion, or endocytosis. The availability of fluorescently labeled apoB proteins will facilitate the exploration of the assembly, degradation, and transport of Blps and help to identify novel compounds that interfere with these processes via high-throughput screening.

Gross saponin of Tribulus terrestris improves erectile dysfunction in type 2 diabetic rats by repairing the endothelial function of the penile corpus cavernosum

OBJECTIVE

To investigate the effect of gross saponins of Tribulus terrestris (GSTT) on erectile function in rats resulting from type 2 diabetes mellitus (T2DMED).

METHODS

The T2DMED model was constructed by high-fat and high-sugar feeding and streptozotocin injection. After 4 weeks of GSTT intervention. Intracavernous pressure (ICP) and mean arterial pressure (MAP) were measured in each group. The level of nitric oxide (NO) in the cavernous tissue was detected using the nitrate reductase method. The production of reactive oxygen species (ROS) was detected using DHE fluorescent probe detection. Cyclic adenosine monophosphate (cGMP) level was detected by enzyme-linked immunosorbent assay, and endothelial nitric oxide synthase (eNOS) was detected using immunohistochemistry. Masson staining was used to detect the cavernosal smooth muscle/collagen ratio. Apoptosis in endothelial cells was measured using TUNEL. Western blotting method to detect the protein expression level of eNOS, TIMP-1, cleaved caspase 3, and cleaved caspase 9.

RESULTS

After treatment, the ICP and ICP/MAP values of the GSTT were significantly higher than those of the T2DMED group (P<0.05). Unlike the T2DMED group, the GSTT group showed significantly increased NO levels (P<0.05) and decreased ROS levels (P<0.05). There was no significant difference between the GSTT group and the sildenafil group in increasing cGMP levels (P>0.05), and the mixed group had higher levels than these two groups (P<0.05). Immunohistochemistry and Western blotting showed that the expression of eNOS in the GSTT was significantly higher than that in the T2DMED groups (P<0.05). Masson staining showed that the smooth muscle/collagen ratio of the GSTT group was significantly higher than that of the T2DMED groups (P<0.05), the expression of TIMP-1 was lower than that of T2DMED group (P<0.05). TUNEL assay showed that the apoptotic index and cleaved caspase 3 and cleaved caspase 9 expression level of GSTT group were lower than that of the T2DMED group (P<0.05).

CONCLUSION

GSTT can protect T2DMED rats' erectile function by improving penile endothelial function and inhibiting cavernosum fibrosis, inhibiting apoptosis, and is synergistic with sildenafil.

Targeting microsomal triglyceride transfer protein and lipoprotein assembly to treat homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (HoFH) is a polygenic disease arising from defects in the clearance of plasma low-density lipoprotein (LDL), which results in extremely elevated plasma LDL cholesterol (LDL-C) and increased risk of atherosclerosis, coronary heart disease, and premature death. Conventional lipid-lowering therapies, such as statins and ezetimibe, are ineffective at lowering plasma cholesterol to safe levels in these patients. Other therapeutic options, such as LDL apheresis and liver transplantation, are inconvenient, costly, and not readily available. Recently, lomitapide was approved by the Federal Drug Administration as an adjunct therapy for the treatment of HoFH. Lomitapide inhibits microsomal triglyceride transfer protein (MTP), reduces lipoprotein assembly and secretion, and lowers plasma cholesterol levels by over 50%. Here, we explain the steps involved in lipoprotein assembly, summarize the role of MTP in lipoprotein assembly, explore the clinical and molecular basis of HoFH, and review pre-clinical studies and clinical trials with lomitapide and other MTP inhibitors for the treatment of HoFH. In addition, ongoing research and new approaches underway for better treatment modalities are discussed.

Iron loading impairs lipoprotein lipase activity and promotes hypertriglyceridemia

Iron loading is associated with altered lipid metabolism, but underlying mechanisms remain unknown. We compared serum iron and triglycerides (TGs) in Belgrade rats, a genetic model of iron-loading anemia. Homozygous b/b rats had greater serum iron (68 vs. 28 μM; P=0.0004) and TG levels (180 vs. 84 mg/dl; P=0.014) compared to +/b controls. To confirm the association between iron loading and high TGs, Fischer rats were fed chow containing 1% carbonyl iron. Compared to controls pair-fed normal chow, carbonyl iron-fed rats had elevated serum iron (42 vs. 21 μM; P=0.007) and TGs (190 vs. 115 mg/dl; P=0.009). Despite normal hepatic production and secretion, TG clearance was lower in b/b than +/b rats due to reduced serum lipoprotein lipase (LPL) activity (3.1 vs. 5.0 mM/min; P=0.026). Likewise, LPL was lower in carbonyl iron-fed rats compared to controls (2.4 vs. 3.7 mM/min; P=0.017). Direct addition of iron to serum ex vivo or recombinant LPL in vitro decreased enzymatic activity in a dose-dependent manner. Lowering serum iron in Belgrade rats reduced TG levels (274 to 67 mg/dl, P=0.001). This study explains the relationship between iron status and lipid metabolism and provides mechanistic support for interventions that reduce serum iron levels in individuals at risk for hypertriglyceridemia.

Apolipoprotein B-containing lipoprotein assembly in microsomal triglyceride transfer protein-deficient McA-RH7777 cells

Microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apolipoprotein (apo) B-containing lipoproteins. Previously, we demonstrated that the N-terminal 1,000 residues of apoB (apoB:1000) are necessary for the initiation of apoB-containing lipoprotein assembly in rat hepatoma McA-RH7777 cells and that these particles are phospholipid (PL) rich. To determine if the PL transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB:1000-containing lipoproteins, we employed microRNA-based short hairpin RNAs (miR-shRNAs) to silence Mttp gene expression in parental and apoB:1000-expressing McA-RH7777 cells. This approach led to 98% reduction in MTP protein levels in both cell types. Metabolic labeling studies demonstrated a drastic 90-95% decrease in the secretion of rat endogenous apoB100-containing lipoproteins in MTP-deficient McA-RH7777 cells compared with cells transfected with negative control miR-shRNA. A similar reduction was observed in the secretion of rat endogenous apoB48 under the experimental conditions employed. In contrast, MTP absence had no significant effect on the synthesis, lipidation, and secretion of human apoB:1000-containing particles. These results provide strong evidence in support of the concept that in McA-RH7777 cells, acquisition of PL by apoB:1000 and initiation of apoB-containing lipoprotein assembly, a process distinct from the conventional first-step assembly of HDL-sized apoB-containing particles, do not require MTP. This study indicates that, in hepatocytes, a factor(s) other than MTP mediates the formation of the PL-rich primordial apoB:1000-containing initiation complex.

Identification of domains in apolipoprotein B100 that confer a high requirement for the microsomal triglyceride transfer protein

The microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apoB-containing lipoproteins. To investigate the role of MTP in lipoprotein assembly, we determined the ability of carboxyl-terminally truncated forms of apoB to be secreted from cells treated with the MTP inhibitor 4'-bromo-3'-methylmetaqualone (Benoist, F., Nicodeme, E., and Grand-Perret, T. (1996) Eur. J. Biochem. 240, 713-720). In Caco-2 and mhAT3F cells that produce apoB100 and apoB48, the inhibitor preferentially blocked apoB100 secretion. When the inhibitor was tested on McA-RH7777 cells stably transfected with cDNAs encoding human apoB100, apoB72, apoB53, apoB29, and apoB18, the secretion of apoB100, apoB72, and apoB53 was preferentially impaired relative to apoB48 and shorter forms. To delineate the region between apoB48 and apoB53 that has a high requirement for MTP, we used puromycin to generate a range of truncated forms of apoB in HepG2 cells. The secretion of apoB53 and longer forms of apoB was markedly affected by low concentrations of the MTP inhibitor (approximately 1 microM), whereas apoB51 and smaller forms of apoB were only affected at higher concentrations (> 10 microM). The size-related sensitivity to MTP inhibitor was not due to late processing or retention, since the same result was observed when nascent lipoproteins were isolated from the endoplasmic reticulum. The MTP inhibitor did not alter the density of the secreted lipoproteins, indicating that each apoB polypeptide requires a minimally defined amount of lipid to attain a secretable conformation. Our results suggest that the folding of the domain between apoB51 and apoB53 has a high requirement for lipid. This domain is predicted to form amphipathic alpha-helices and to bind lipid reversibly. It proceeds and is followed by rigid amphipathic beta-sheets that are predicted to associate with lipid irreversibly. We speculate that these domains enable apoB to switch from a stable lipid-poor conformation in apoB48 to another lipid-rich conformation in apoB100 during lipoprotein assembly.

Evaluation of cultured hamster hepatocytes as an experimental model for the study of very low density lipoprotein secretion

The secretion of triacylglycerol, cholesterol and cholesteryl ester in very low density lipoprotein (VLDL) by cultured hamster hepatocytes was studied, and the results compared with those obtained previously using cultured rat hepatocytes and the human hepatoma cell line HepG2. The hamster cells secreted apolipoprotein B and VLDL triacylglycerol, cholesterol and cholesteryl ester linearly during 24 h in culture, and this time period was used in all experiments. Addition of oleate (1 mM) to the culture medium resulted in increased secretion of triacylglycerol, but cholesterol ester output were unchanged. Triacylglycerol secretion was also increased in the presence of lipogenic substrates (10 mM lactate + 1 mM pyruvate) plus dexamethasone (1 microM), but not with either of these agents alone. Inhibition of cholesterol synthesis in the hamster cells by incubation with mevinolin (2 micrograms/ml) did not change VLDL lipid secretion, but stimulation using mevalonate lactone resulted in decreased triacylglycerol output. Manipulation of the rate of cholesterol esterification in the hepatocytes by inhibiting or stimulating the activity of acyl coenzyme A cholesterol:acyl transferase using the inhibitor Dup128 (25 microM) and 25-hydroxycholesterol (50 microM), respectively, had no effect on the secretion of VLDL lipid. In the presence of 1 mM oleate plus 25-hydroxycholesterol, however, a rise in the output of triacylglycerol and cholesteryl ester was observed. Hepatocytes prepared from hamsters fed 2% cholestyramine secreted significantly less triacylglycerol than those from animals given the control diet, but cholesterol and cholesteryl ester output were unchanged, despite a decrease of about 40% in the total cholesterol content of the cells. These results show that the secretion of lipid in VLDL in hamster hepatocytes differs from that in rat and human liver in its response to dietary cholestyramine, and from rat hepatocytes and HepG2 cells in its response to changes in the rate of lipogenesis and cholesterol synthesis and esterification. Overall, hamster hepatocytes appear to be less susceptible to modification the rate of hepatic VLDL secretion, and should provide a useful additional tool for the investigation of this process.

Hyperlipidemia in streptozocin-diabetic hamsters as a model for human insulin-deficient diabetes: comparison to streptozocin-diabetic rats

Characteristics of the lipoprotein profile and metabolism of triglyceride-rich lipoproteins in diabetic hamsters were investigated to assess their suitability as a model for human diabetic hyperlipidemia. Diabetes was induced in the hamsters by intraperitoneal injection of streptozocin (30 mg/kg) for 3 days and compared with the results in streptozocin-diabetic rats (50 mg/kg intravenously). Similar degrees of hyperglycemia and hypoinsulinemia were observed 8 to 10 days after the final streptozocin injection in both groups. Fasting plasma lipid concentrations were about 2.5 times greater in hamsters than in rats. Plasma cholesterol was principally associated with high-density lipoprotein (HDL) in both rodents, although the distribution in very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) was significantly greater in hamsters (44%) than in rats (13%). Diabetes increased the concentrations of triglyceride, cholesterol, and phospholipid 5.6- to 7.8-fold in hamsters, whereas it increased them only 1.3- to 1.6-fold in rats. Diabetic hamsters have a plasma lipoprotein profile similar to that of diabetic man, ie, triglyceride-rich lipoproteins are increased and HDL cholesterol is decreased. The concentration of HDL cholesterol was inversely correlated with the severity of hypertriglyceridemia (r = .76, P < .005). This combination of events does not occur in diabetic rats. Hamsters had a low level of apoprotein B-48-containing triglyceride-rich lipoproteins, although diabetes increased the estimated concentration by fourfold. In rats apoprotein B-48 is the predominant form, but diabetes did not alter the relative proportion of apoprotein B isoforms.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

Lipid and lipoprotein synthesis in isolated and cultured hepatocytes from lean and obese Zucker rats

Hepatocytes were isolated by EDTA perfusion of livers from lean (Fa/-) and obese (fa/fa) Zucker rats. Triacylglycerol (TG) and sn-glycerol 3-phosphate were increased in fa/fa hepatocytes, but free fatty acids, cholesterol and phospholipid concentrations were similar in both groups. In spite of an identical fatty acid uptake rate, glycerolipid synthesis was higher in obese compared to lean rat hepatocytes, and this difference remained for at least 2-3 days of culture. Triacylglycerol mass secretion was 2-fold higher in obese than in lean rat hepatocytes. This was confirmed by the higher incorporation of labeled glycerol and oleic acid into the medium TG fraction floating at density 1.006 g/ml. Density gradient ultracentrifugation of [14C]oleate-labeled lipoproteins showed that fa/fa hepatocytes secreted more TG-rich lipoproteins, and that 87% of the label was in the VLDL fraction compared with 67% in the medium of Fa/- hepatocytes. Decreased utilisation of leucine for protein synthesis in obese rat compared to lean rat hepatocytes was associated with enhanced leucine oxidation to CO2. [35S]Methionine incorporation showed an identical cell protein synthesis rate. Autoradiography after PAGE separation of secreted apolipoproteins (apoBh, Bl, apoA-VI, apoE, apoA-I, apoC) showed an identical pattern in both cell types.

Synthesis, modification, and flotation properties of rat hepatocyte apolipoproteins

We have studied apolipoprotein synthesis, intracellular modification and secretion by primary adult rat hepatocyte cultures using continuous pulse or pulse chase labeling with [35S]methionine, immunoprecipitation and two-dimensional isoelectric focusing/polyacrylamide gel electrophoresis. The flotation properties of the newly secreted apolipoproteins were studied by discontinuous density gradient ultracentrifugation and one- and two-dimensional polyacrylamide gel electrophoresis. These studies showed that rat hepatocyte apoE is modified intracellularly to produce minor isoproteins that differ in size and charge. One of these minor isoproteins represents a monosialated apoE form (apoE3s1). Similarly, apoCIII is modified intracellularly to produce a disialated apoCIII form (apoCIIIs2), whereas newly synthesized apoA-I and apoA-IV are not glycosylated and overlap on two-dimensional gels with the proapoA-I and the plasma apoA-IV form, respectively. Both unmodified and modified apolipoproteins are secreted into the medium. Separation of secreted apolipoproteins by density gradient ultracentrifugation has shown that 50% of apoE, 80% of apoA-I, and more than 90% of apoA-IV and apoCIII are secreted in a lipid-poor form, whereas apoB-100 and apoB-48 are 100% associated with lipids. ApoB-100 floats in the VLDL and IDL regions, whereas apoB-48 is found in all lipoprotein fractions. ApoE and small amounts of apoA-I, apoA-IV and apoCIII float in the HDL region. Small amounts of apoE and apoCIII are also found in the VLDL and IDL regions, and apoE in the LDL region. Ultracentrifugation of nascent lipoproteins in the presence of rat serum promoted flotation of apoA-I and apoA-IV in the HDL fraction and resulted in increased flotation and distribution of apoE and apoCs in VLDL, IDL and LDL regions. These observations are consistent with the hypothesis that intracellular assembly of lipoproteins involves apoB-48 and apoB-100 forms, whereas a large portion of apoA-I, apoCIII and apoA-IV can be secreted in a lipid-poor form, which associates extracellularly with preexisting lipoproteins.

Rat intestinal apolipoprotein B gene expression. Evidence for integrated regulation by bile salt, fatty acid, and phospholipid flux

We previously reported that intestinal apo B48 synthesis in the rat was unaltered by dietary triglyceride intake but demonstrated regulation in response to biliary lipid availability. Studies are now presented in which the mechanisms underlying biliary lipid dependent expression of intestinal apo B48 synthesis have been investigated further. Bile salt replacement was effective in a dose- and structure-dependent manner in reexpressing intestinal apo B48 synthesis after prolonged bile diversion. Further experiments suggested that this effect of bile salt may be related to facilitated uptake of fatty acid. A role for mucosal phospholipid flux was suggested by studies in which infusion of lysolecithin, with or without Na taurocholate, produced complete reexpression of apo B48 synthesis in jejunal enterocytes. Over a four- to sixfold range of apo B48 synthesis rates in both jejunum and ileum, there was no change in apo B mRNA size or abundance as determined by RNA blot hybridization. Analysis of both intestinal mucosa and microsome lipid content in a variety of settings revealed that apo B48 synthesis rates were correlated with microsome triglyceride fatty acid content (r = 0.65, P less than 0.005) but not free fatty acid or phospholipid content. These studies demonstrate a physiologic role for elements of biliary lipid flux in the regulation of apo B gene expression. The data suggest that an integrated mechanism may exist whereby apo B48 synthesis is related to microsome triglyceride flux, particularly at low levels of lumenal substrate availability.

Effects of oleic acid on the biosynthesis of lipoprotein apoproteins and distribution into the very-low-density lipoprotein by the isolated perfused rat liver

The effects of oleic acid on the biosynthesis and secretion of VLDL (very-low-density-lipoprotein) apoproteins and lipids were investigated in isolated perfused rat liver. Protein synthesis was measured by the incorporation of L-[4,5-3H]leucine into the VLDL apoproteins (d less than 1.006) and into apolipoproteins of the whole perfusate (d less than 1.21). Oleate did not affect incorporation of [3H]leucine into total-perfusate or hepatic protein. The infusion of oleate, however, increased the mass and radioactivity of the VLDL apoprotein in proportion to the concentration of oleate infused. Uptake of oleate was similar with livers from fed or fasted animals. Fasting itself (24 h) decreased the net secretion and incorporation of [3H]leucine into total VLDL apoprotein and decreased the output of VLDL protein by the liver. A linear relationship existed between the output of VLDL triacylglycerol (mumol/h per g of liver) and secretion and/or synthesis of VLDL protein. Net output of VLDL cholesterol and phospholipid also increased linearly with VLDL-triacylglycerol output. Oleate stimulated incorporation of [3H]leucine into VLDL apo (apolipoprotein) E and apo C by livers from fed animals, and into VLDL apo Bh, B1, E and C by livers from fasted rats. The incorporation of [3H]leucine into individual apolipoproteins of the total perfusate lipoprotein (d less than 1.210 ultracentrifugal fraction) was not changed significantly by oleate during perfusion of livers from fed rats, suggesting that the synthesis de novo of each apolipoprotein was not stimulated by oleate. This is in contrast with that observed with livers from fasted rats, in which the synthesis of the total-perfusate lipoprotein (d less than 1.210 fraction) apo B, E and C was apparently stimulated by oleate. The observations with livers from fed rats suggest redistribution of radioactive apolipoproteins to the VLDL during or after the process of secretion, rather than an increase of apoprotein synthesis de novo. It appears, however, that the biosynthesis of apo B1, Bh, E and C was stimulated by oleic acid in livers from fasted rats. Since the incorporations of [3H]leucine into the VLDL and total-perfusate apolipoproteins were increased in fasted-rat liver when the fatty acid was infused, part of the apparent stimulated synthesis of the VLDL apoprotein may be in response to the increased formation and secretion of VLDL lipid.

Synthesis and secretion of lipoproteins by human hepatocytes in culture

Confluent monolayers of normal human hepatocytes obtained by collagenase perfusion of liver fragments were incubated in a serum-free medium. Intracellular apolipoproteins apo AI, apo C, apo B, and apo E were detected between Day 1 and Day 6 of the culture by immunoenzymatic staining using polyclonal antibodies directed against these apoproteins and monoclonal antibodies directed against both forms of apo B (B100 and B48). Translation of mRNA isolated from these hepatocytes in an acellular system revealed that apo AI and apo E were synthesized as the precursor forms of mature plasma apo AI and apo E. Three lipoprotein fractions corresponding to the density of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) were isolated from the medium at Day 5 of culture and examined by electron microscopy after negative staining. VLDL and LDL particles are similar in size and shape to plasma lipoproteins; spherical HDL are larger than normal plasma particles isolated at the same density. Their protein represented 44, 19.5, and 36.5% respectively, of the total lipoprotein protein. The secretion rate of VLDL protein corresponded to that measured in primary cultures of rat hepatocytes. After incorporation of [3H]glycerol, more than 92% of the [3H]triglyceride secreted into the medium was recovered in the VLDL fraction. These results demonstrate that primary cultures of normal human hepatocytes are able to synthesize and secrete lipoproteins and thus could be a useful model to study lipoprotein metabolism in human liver.

The Fao cell. A tissue culture model for lipoprotein synthesis and secretion. II. Modulation by lipid depletion and supplementation

We have shown that the Fao cell, a differentiated rat hepatoma line, is an excellent model for the study of the synthesis of lipoproteins (Scarino, M L & Howell, K E, Exp cell res 170 (1987) 1 [1]. Here we demonstrate that variation of the lipid composition of the growth medium significantly modulates the composition and quantity of particles formed. Three growth conditions were compared: normal, lipid-depleted, and lipid-supplemented. The synthesis of both the protein and lipid moieties of the lipoproteins was quantitated using the radioactive metabolic precursors [35S]methionine and [14C]acetate. The total secretion of the cells was collected and fractionated into four density classes equivalent to plasma lipoproteins and a bottom fraction equivalent to plasma proteins. Each density class was evaluated for the apoprotein distribution after separation by SDS-PAGE and for lipid distribution and composition after lipid extraction. ApoE accounts for approx. 15% of the total protein synthesized and is the major apoprotein. The amount synthesized remains relatively constant under all growth conditions. In contrast, the amount of apoB synthesis varies over 600-fold. In lipid-depleted conditions, only 0.01 times the normal amount was synthesized, while in lipid-supplemented conditions 6.2 times the normal amount was synthesized. ApoB was associated with the lighter fraction; therefore the modulation increased the quantity of low-density particles formed. A similar but far less pronounced variation of the heavier particles and the apoA-I concentration was obtained. Under lipid-depleted conditions, 0.75 times the normal amount was synthesized, while under lipid-supplemented conditions 2.6 times the normal quantity was synthesized.

The serum lipoprotein pattern of water buffalo (Bubalus bubalis)

1. The serum lipoprotein pattern of water buffalo was studied by means of electrophoresis and the lipoproteins were isolated by ultracentrifugation on the basis of their hydrated density. 2. High density lipoproteins (HDL) showed a higher level of cholesterol than did the other lipoproteins. Moreover, the level of phospholipids was higher in HDL than in very low density lipoproteins (VLDL). 3. The buffalo B100 apoprotein was similar to that of man and rat. Three apoproteins similar to human apo E, apo AI and AII were found in buffalo HDL, buffalo VLDL contained essentially apo B protein.

Insulin effects on apolipoprotein B lipoprotein synthesis and secretion by primary cultures of rat hepatocytes

Lipoprotein synthesis and secretion were examined in primary cultures of rat hepatocytes cultured on collagen-coated plates and incubated with pharmacologic and physiologic concentrations of insulin. Media insulin concentration declined rapidly over the course of incubation indicating that hepatocytes rapidly degrade insulin. When insulin was present in the media, cellular triglyceride accumulated while lipid secretion declined. Insulin inhibited the incorporation of labeled amino acids into total secretory lipoprotein apoproteins and apolipoprotein B (apo B) as well as apo B mass as measured by monoclonal radioimmunoassay. The effect of insulin on apo B secretion occurred as early as three hours after the addition of insulin to the culture media and both apo B of higher molecular weight (apo BH) and apo B of lower molecular weight (apo BL) were affected. Cellular apo B did not accumulate within cells. The majority of secretory lipid radioactivity synthesized from acetate was in VLDL density lipoproteins. The composition of newly synthesized lipids as assessed by thin layer chromatography was not significantly altered with insulin. These studies support the finding that insulin inhibits VLDL secretion by hepatocytes while at the same time stimulating overall triglyceride synthesis. A suggested mechanism is that insulin uncouples triglyceride and apo B synthesis, which influences subsequent lipoprotein assembly and secretory pathways. These results are consistent with the concept that postprandial insulin release inhibits hepatic lipoprotein secretion while intestinal lipoprotein metabolic pathways are most active.

Tissue-specific expression and developmental regulation of the rat apolipoprotein B gene

Expression of the apolipoprotein B (apoB) gene was examined in a variety of fetal, neonatal, and adult rat tissues by probing RNA blots with a cloned rat apoB cDNA. Among 10 adult male tissues surveyed, small intestine had the highest concentration of apoB mRNA. Its abundance in liver and adrenal gland was 40% and 0.5%, respectively, of that in small bowel, while none was detected in colon, kidney, testes, spleen, lung, heart, or brain. ApoB mRNA is as abundant in 18-day fetal liver as at any subsequent period of hepatic development. In contrast, the concentration of apoB mRNA remains low in fetal intestine until the last (21st) day of gestation, when it increases sharply to levels that are several-fold higher than in the liver. ApoB mRNA levels in fetal membranes harvested during this late gestational period were 10 times greater than in fetal liver. Since the major lipoprotein species in 19-day fetal plasma is low density lipoprotein, these observations suggest that fetal liver, and particularly its functional homologue, the yolk sac, are the principal sites of fetal lipoprotein synthesis at this stage of development. A 20-fold increase in placental apoB mRNA concentrations during the last 48 hr of pregnancy (to a level that is 50% of that encountered in fetal membrane RNA) suggests a specific role for this organ in maternal-fetal lipid transport immediately prior to parturition. Pulse-labeling experiments using 21-day fetal tissue slices showed that the liver synthesizes both apoB-100 (B-PI) and apoB-48 (B-PIII) albeit in somewhat different ratios than the adult organ. Fetal intestine produces almost exclusively the smaller apoB species, while fetal membranes and placenta synthesize only the larger peptide. The postnatal pattern of apoB mRNA accumulation is similar in liver and intestine. Profound decreases were observed during the late suckling and weaning periods, followed by an increase to adult levels. These final concentrations were similar to those encountered at birth. Analysis of these developmental changes offers an opportunity to generate testable hypotheses about the factors that modulate apoB synthesis.

Human apolipoprotein B: identification of cDNA clones and characterization of mRNA

Apolipoprotein B (apoB) is a major protein component of low density and very low density lipoproteins. Because of its large size and heterogeneity, molecular studies of apoB have been difficult, and its structure and regulation remain poorly understood. We now report the identification of human apoB cDNA clones by antibody screening of hepatoma libraries in the expression vector lambda gt11. Both oligo(dT) primed and random primed libraries were constructed and screened with polyclonal antibodies to intact apoB, as well as with antibodies raised against a synthetic peptide based on the limited amino acid sequence available for apoB. The identity of the clones was unambiguously established by comparisons of the cloned cDNA sequences with apoB amino acid sequences. The clones hybridize to an exceptionally large 20 kb mRNA that is present in liver and intestine but not other tissues examined, consistent with the distribution expected from protein biosynthetic studies. The properties of the mRNA have implications for the biogenesis of the multiple apoB molecular weight forms secreted by liver and intestine.

Apolipoprotein B synthesis in humans: liver synthesizes only apolipoprotein B-100

Apolipoprotein (apo) B-100 and B-48 are prominent apolipoproteins in VLDL, IDL, and chylomicrons. Organ cultures of normal adult human liver were established to ascertain the form of apo B synthesized by hepatocytes in humans. Human liver was minced and incubated in 15 mL methionine-free RPMI-1640 medium with 10% dialyzed fetal calf serum plus 250 microCi 35S-methionine for eight hours at 37 degrees C. Lipoproteins secreted by the liver were isolated by ultracentrifugation and the content of newly synthesized apo B determined by quantitation of radioactivity in the apoB-100 and apoB-48 bands after separation by 3% NaDodSO4 gel electrophoresis. In the eight-hour period, 2.5% to 3.2% of added 35S-methionine was secreted in TCA-precipitable protein of which 0.34% was apo B. Ninety-nine percent of the apo B in VLDL, IDL, and LDL was in the apo B-100 electrophoretic band. No significant radioactivity was detected in the apo B-48 electrophoretic band. Eighty-nine percent of the total radioactivity of apo B-100 was in VLDL with 3% and 8% in IDL and LDL, respectively. These results establish that adult human liver in organ culture synthesizes apo B-100 but not apo B-48.

Cloning and expression of apolipoprotein B, the major protein of low and very low density lipoproteins

We report the cloning of cDNAs for rat liver apolipoprotein B (apo B) and the use of the cloned sequences to examine apo B expression at the level of mRNA in rat tissues. Fifteen putative apo B clones were identified by antibody screening of a rat liver cDNA library in the lambda gt11 expression vector. The identity of the clones was confirmed by immunological studies of the fusion protein products. All clones appear to contain sequences found only in apo B PI, the high molecular weight form of rat liver apo B. Blotting studies show that the clones hybridize to a single 20-kilobase liver mRNA species, sufficiently large to encode the entire apo B PI peptide, which is estimated to be 400 kDa in size. Apo B PI mRNA is abundant in liver and present in lower amounts in intestine but is absent in a variety of other tissues examined. This tissue distribution is consistent with that expected from studies on the in vivo synthesis of apo B. One clone, corresponding to a 240-base segment of the apo B PI mRNA, was sequenced and found to exhibit homology with a short region of rat apo E mRNA. Analysis of the secondary structure of the corresponding peptide did not show the preponderance of amphipathic alpha-helical structures characteristic of other apolipoproteins examined thus far.

Dietary cholesterol does not affect the synthesis of apolipoproteins B and E by rat hepatocytes

To examine the unproved hypothesis that dietary cholesterol affects the synthesis of apolipoprotein B and E, we fed rats a cholesterol-rich diet that has been shown to alter dramatically the serum concentrations of these apolipoproteins. Rats fed for 4 weeks on a cholesterol-rich diet accumulate increased concentrations of low Mr apolipoprotein B (+2.7-fold) and decreased concentrations of apolipoprotein E (-40%) in their serum. Hepatocytes obtained from similarly treated rats were placed in monolayer culture and the rate of synthesis de novo of apolipoproteins was determined. Although cells from cholesterol-fed rats remained filled with lipid droplets throughout the experimental period, there was no difference in plating efficiency or viability, compared with cells obtained from chow-fed control rats. Both groups of cells synthesized and secreted immunoprecipitable apolipoproteins B and E at similar rates throughout the 18 h experiment. Thus there was a discordance between the effects of dietary cholesterol on serum apolipoprotein concentrations and hepatocyte synthesis and secretion. The data indicate that altered hepatic apolipoprotein synthesis cannot account for the changes in serum apolipoprotein concentrations caused by dietary cholesterol.

Apoprotein B-48 is a constant finding in very low density lipoproteins of humans

The results of this study indicate that very low density lipoprotein (VLDL) from the plasma of fasting normolipidemic or slightly hypertriglyceridemic subjects contains two apo B species. In SDS gel electrophoresis, the VLDL shows the presence of a major band corresponding to low density lipoprotein (LDL) apo B (apo B-100) and a minor band with the appropriate mobility of the lymph chylomicron apo B (apo B-48). The reactivity of monoclonal antibodies directed against opportunely selected human apo B suggests that the protein with the lower molecular weight corresponds to apo B-48. This finding was confirmed by using immunoadsorbants and affinity chromatography with monoclonal antibodies that react only with apo B-100. Through this method, VLDL were separated into two fractions: one that had only apo B-100 and one with both apo B-100 and apo B-48. Hepatic and intestinal VLDL may constitute different particles. The ratio of apo B-100 to apo B-48 in VLDL decreased as the mass of fasting plasma VLDL increased. This may improve our understanding of the metabolism of triglyceride-rich lipoproteins. The investigation of the new subspecies of apo B may be relevant in understanding the atherogenetic process and better defining the hyperlipidemic states.